1. Field of the Invention
The present invention relates to a negative electrode for an alkaline secondary cell and an alkaline secondary cell using this negative electrode.
2. Description of the Related Art
One of well-known alkaline secondary cells is a nickel-hydrogen secondary cell. The nickel-hydrogen secondary cell has a higher capacity than a nickel-cadmium secondary cell and is environmentally safe. For these reasons, the nickel-hydrogen secondary cell offers versatile applications in a variety of portable devices, electric hybrid cars, etc. Because of its versatility, the nickel-hydrogen secondary cell is required to be improved in properties including a cycle life property.
A nickel-hydrogen secondary cell that is known for an improved cycle life property is the one disclosed, for example, in Patent Document 1 (Unexamined Japanese Patent Publication (Kokai) No. 2009-206004). This nickel-hydrogen secondary cell contains water-repellent fluorine oil in the negative electrode containing hydrogen-storage alloy. The fluorine oil is partially applied onto the surface of the hydrogen-storage alloy and moderately limits contact between the hydrogen-storage alloy and alkaline electrolyte. This prevents the hydrogen-storage alloy from being deteriorated due to oxidation caused by the alkaline electrolyte when the nickel-hydrogen secondary cell containing fluorine oil is repeatedly charged and discharged. The nickel-hydrogen secondary cell of Patent Document 1 is thus improved in cycle life property.
The fluorine oil is mixed and kneaded with hydrogen-storage alloy powder, a conducting agent, etc., in a process of producing negative slurry. Over the course of the kneading process, the fluorine oil adheres to the surface of the hydrogen-storage alloy.
The fluorine oil simply physically adheres to the surface of the hydrogen-storage alloy and is therefore easy to flow and spread over a relatively wide area of the hydrogen-storage alloy surface during the slurry-kneading process. The fluorine oil consists of water-repellent molecules. For this reason, a water-repellent film made of the fluorine oil is formed over a relatively wide area of the hydrogen-storage alloy surface. When the hydrogen-storage alloy surface is protected by the film over the relatively wide area, the hydrogen-storage alloy is adequately inhibited from being oxidized. Consequently, a cell containing the hydrogen-storage alloy with the film has an excellent cycle life property.
However, if the hydrogen-storage alloy surface is covered with fluorine oil over a wide area, the hydrogen-storage alloy is limited more than necessary in contact with the alkaline electrolyte. In the nickel-hydrogen secondary cell containing the hydrogen-storage alloy that is limited in contact with the alkaline electrolyte, cell reaction is not promoted especially under a low-temperature environment. On this account, conventional nickel-hydrogen secondary cells provided with a negative electrode containing fluorine oil are noticeably degraded in discharge property under a low-temperature environment (hereinafter, referred to as a low-temperature discharge property). One idea for improving the low-temperature discharge property of the conventional nickel-hydrogen secondary cells is to reduce the additive amount of fluorine oil. This, however, shortens the cycle life of the cells.
As seen above, in respect of the conventional nickel-hydrogen secondary cells, the low-temperature discharge property has to be sacrificed in some measure to enhance the cycle life property. On the other hand, in order to enhance the low-temperature discharge property, the cycle life property needs to be sacrificed to some extent. In other words, it is difficult to achieve a balance between the enhancement of the cycle life property and that of the low-temperature discharge property of the cells.